In the injection molding of polyolefin battery cases, the interior partitions in the case are formed by elongated, cantilever mounted cores extending into the mold cavity. Because of the irregular flow of the hot melt and high injection pressure in the injection process, undesirable shifting of the cantilever mounted cores can occur causing irregular, non-uniform thickness of the partition walls. A recognized means for locking the cores against this undesirable shifting by placing the cores in transverse compression is disclosed and claimed in Wilds et al U.S. Pat. No. 3,473,197. The solution to the problem described in the patent has been satisfactory, but inherently produces a series of wedge-shaped protrusions on the inner face of the battery case base wall. The protrusions were acceptable because, until recently, polyolefin battery cases were designed with a series of spacer abutments or "mud ribs" extending from the inner surface of the base wall of the case. These functioned to space the packets of battery plates, inserted in the case, from the base allowing room for unwanted particles such as lead "burns" dropping from the plates to settle without shorting the plates. Since the wedge-shaped protrusions, caused by the locking pins, were below the profile of the spacer ribs or abutments, they created no difficulty. More recently, because of improved forms of packaging for the packets of battery plates, the use of spacer ribs has become unnecessary and designers of battery cases have required "smooth-bottomed" configurations so that the space formerly defined by the ribs can be utilized by battery plates of increased size. The inwardly projecting protrusions inherent in the core-locking means of U.S. Pat. No. 3,473,197 have thus become unsuitable in smooth-bottomed battery cases. While the structure disclosed in Sullivan et al, U.S. Pat. No. 3,767,156 produces a battery case having fewer protrusions in the base of the battery case, the cores are not held or locked by members engaging the cores and applying transverse, compressive stress on the cores, a necessary feature if the cores are to be effectively locked in place.
The concept of the present invention envisages a core-locking strategy in which the cores are locked in lateral compression by means, taking the form of locking pins, located within the cores themselves and movable transversely to the longitudinal axis of the cores to lock the adjoining cores in transverse compression thereby holding the cores against displacement during a portion of the injection process. No locking pins extending from the cavity are necessary, and flat or smooth bottomed cases can thus be molded. Since the locking pins are supported within, and extend from, the side surfaces of the cores themselves, the core position-locking engagement of transversely extending pins with the side walls of the adjacent cores in a parallel array of cores can be accomplished. While locking of a single core by pins engaging the core's sidewall is known in the prior art as disclosed in British Pat. Nos. 1,090,810 (1967) and 599,570 (1948), since these pins are disposed and actuated from positions exterior to the core, the arrangement cannot be utilized to extend between, or place in transverse compression, each core in an array of closely spaced, parallel cores. While alternate forms of driving mechanism for the locking pins are disclosed herein, it will be understood that within-the-cores placement of the pins and locking drives represents the primary distinguishing feature of the present invention.